1. Field of the Invention
The invention relates to the field of atom probe field ion microscopy and in particular for application to materials that can not be prepared for atom probe field ion microscopy using electropolishing.
2. Description of the Prior Art
The frontiers of geology and material science are constantly pushing the limits of detection of elemental distributions to ever finer scales. Atom probe field ion microscopy (APFIM) is a highly attractive technique in these fields despite the inherent difficulties in analyzing semiconducting and insulating materials. The local electrode atom probe (LEAP) shows potential for overcoming some of the difficulties in analyzing low conductivity samples. The National Aeronautics and Space Administration (NASA) is developing a prototype miniature local electrode atom probe (Mini-LEAP) at the Jet Propulsion Laboratory (JPL) for in-situ implementation in remote and extraterrestrial environments.
In conjunction with the development of the Mini-LEAP, a conventional APFIM study of several poorly conducting materials, has been initiated. Previous attempts to study rutile (TiO2) and magnetite (Fe3O4) have shown how difficult samples of poorly conducting materials are to prepare for APFIM analysis.
Sharp APFIM specimens are generally produced using straightforward electropolishing methods. However, metallic multilayered samples and samples with specific geometries are much more difficult to prepare using electropolishing. The focused ion beam (FIB) was first used by Larsen et. al. in 1998 to trim multilayered metallic samples for APFIM using line milling at low angles to the axis of the specimen. Later, in 1999 an annular milling pattern centered on the axis of the specimen was used by Larsen to mill sharp, cylindrical specimens of these metallic multilayers.
Camus, Melmed and Banfield introduced the method of sharp shards (MSS) in 1991 for preparing samples that are not amenable to chemical etching. Sharp shards alone proved insufficient for successful APFM and the technique was abandoned. Kuhlman, et al. (2001) took advantage of the advances in FIB milling in combination with the MSS to produce successful APFIM specimens of poorly conducting magnetite (Fe3O4). The current work takes advantage of advances in microelectronics fabrication to more precisely shape and sharpen poorly conducting materials into APFIM specimens. These specimens require far less FIB milling, a time-intensive and expensive procedure.
The prior method is called the method of sharp shards because it involves crushing the material of interest and selecting microscopic sharp shards of the material for use as specimens. Each selected shard is oriented with its sharp tip facing away from the tip of a stainless-steel pin and is glued to the tip of the pin by use of silver epoxy. The MSS method is extended by use of a focused ion beam (FIB) to make the shard very thin (relative to its length) and to make its tip cylindrical and even sharper. The method of sharp shards is extremely time-consuming because the selection of shards must be performed with the help of a microscope, the shards must be positioned on the pins by use of micromanipulators, and the irregularity of size and shape necessitates days of FIB milling to sharpen each shard.